In the process of coking coal in the absence of air, large volumes of gas, commonly called coke oven gas, are produced in addition to the carbonaceous residue. This coke oven gas contains valuable by-products such as ammonia, naphthalene, tar and light oils comprising benzene, toluene and other hydrocarbons which are typically recovered in by-product recovery systems associated with the coke oven plants.
In the by-product recovery processes currently in use, the hot coke oven gas leaves the coking furnaces at temperatures of 600.degree. -700.degree. C. and is shock cooled by a spray of aqueous flushing liquor in a collecting main. This cooling effects a condensation of some of the vapors and removes heavy tar from the coke oven gas. The non-condensed gases at about 75.degree. -80.degree. C. are directed to a primary cooler where further cooling to about 35.degree. -50.degree. C. by spraying with water or ammonia liquor removes additional tar. Part of the ammonia present in the gas is absorbed in the aqueous liquor and, together with the tars and a large portion of naphthalene that are condensed, is carried away with the so-called "ammonia liquor". Any tar remaining in the gas is usually removed in a subsequent electrostatic precipitator.
At this point in the process following the primary coolers and electrostatic precipitators, the gas is contacted with sulfuric acid to remove any remaining ammonia preparatory to its treatment for the recovery of light oils and subsequent scrubbing to eliminate hydrogen sulfide. Although the gas from the ammonia absorber has most of its naphthalene content removed with the tars during the initial cooling of the gas, a significant amount of naphthalene vapor remains in the coke oven gas at this point. Further cooling of the gas is achieved in a final cooler to lower the temperature of the gas stream for efficient processing in the light oil by-product recovery stage.
Cooling of the coke oven gas in the final coolers from about 35.degree. -50.degree. C. to about 20.degree. C. results in the naphthalene vapors precipitating as crystallized solids and is usually accomplished by direct contact of the gas with a cooling liquid. The gas may be passed through a spray of the cooling liquid or it may be bubbled through the liquid. The cooling liquid may be a solvent for naphthalenic materials such as washing oils or benzol oils from the light oil recovery plant. Therefore, in addition to cooling the gas, the precipitated naphthalenic solids are immediately dissolved in these cooling liquids. The liquid effluent from the final cooler must then be stripped of the dissolved solids in additional apparatus to permit recycling of the cooling liquid. Alternatively, the final cooling may be a cold water quench with the precipitated naphthalenic solids being removed from the final cooler as a slurry with the used final cooler water. The solids should be removed prior to recycling the cooling water.
In addition to the naphthalenic solids, a quantity of water vapor also condenses from the coke oven gas and is contaminated with dissolved ammonia, cyanides, sulfides and phenols in dilute concentrations. Accordingly, direct discharge of the once-through cooler water after separation of the naphthalenic solids is not feasible. Furthermore, disposal after removal of the toxic contaminants from the once-through cooling water in a waste water treatment facility is impractical because of the large volume of effluent to be treated. However, recycling of the cooling water results in an accumulation of the contaminants to a concentration that makes treatment of a bleed-stream of the recycling cooling water acceptable.
Because the cooling water that is to be recycled has absorbed heat values from the gas stream and from the condensation of naphthalenic solids and water, it necessarily must be cooled in a heat exchanger after physical separation of suspended naphthalenic solids and prior to reuse in the final cooler. A direct heat exchanger is eliminated as a possibility because toxic contaminants from the water of condensation would be volatilized into the atmosphere. Accordingly, an indirect heat exchanger is the remaining choice.
Nevertheless, such a final cooler system having a recirculating water loop incorporating a solids separator and an indirect heat exchanger does not afford smooth, trouble-free operation. Because the physical separation means cannot completely remove the precipitated naphthalenic solids, there is always some solids suspended in the "clarified" cooling water. Furthermore, the cooling water is saturated with dissolved naphthalenic solids which crystallize out in the indirect heat exchanger as the recycled water is cooled. These suspended and crystallized solids may adhere to the surfaces of the indirect heat exchanger or other equipment possessing flow constrictures clogging the apparatus.
Attempts have been made to solve this problem by recovering the suspended and dissolved naphthalenic solids from the cooling water with bulk quantities of solvent. In U.S. Pat. No. 3,471,999 issued to Sch ,uml/o/ n the cooling water is countercurrently extracted with a solvent for naphthalene. Using large quantities of solvents in a water final cooling system dictates additional equipment and expense to separate the ladened solvent from the water and to strip the solvent from the recoverable naphthalene for reuse.
Therefore, there is a need for a practicable method for the final cooling of coke oven gas using recycled water as the cooling medium.
There is also a need for a method of cooling naphthalene-bearing water with an indirect heat exchanger without clogging.
There is still a need to economically treat naphthalene-bearing water with a solvent to prevent blockage of water conduits in an indirect heat exchanger.